Estimating single molecule conductance from spontaneous evolution of a molecular contact

We present an original method to estimate the conductivity of a single molecule anchored to nanometric-sized metallic electrodes, using a Mechanically Controlled Break Junction (MCBJ) operated at room temperature in liquid. We record the conductance through the metal / molecules / metal nanocontact while keeping the metallic electrodes at a fixed distance. Taking advantage of thermal diffusion and electromigration, we let the contact naturally explore the more stable configurations around a chosen conductance value. The conductance of a single molecule is estimated from a statistical analysis of raw conductance and conductance standard deviation data for molecular contacts containing up to 14 molecules. The single molecule conductance values are interpreted as time-averaged conductance of an ensemble of conformers at thermal equilibrium.Comment: 25 pages, 6 figure

The gaseous extent of galaxies and the origin of Lyman alpha absorption systems. IV: Lyman alpha absorbers arising in a galaxy group

We present new GHRS observations of Lyman alpha absorption lines associated with a group of galaxies towards the QSO 1545+2101. We have identified eight distinct Lyman alpha absorption features in the spectrum of QSO 1545+2101 at a mean redshift of z=0.2648 with a velocity dispersion of 163 km/s. A group of galaxies is detected in the vicinity of this QSO at a mean redshift of z=0.2645 and velocity dispersion 239 km/s. The identification of discrete absorption systems indicates that they arise in clouds of neutral hydrogen rather than in a diffuse intragroup medium. Our analysis suggests that the Lyman alpha absorption lines are associated with individual galaxies in the group, although a one-to-one relationship between absorbers and galaxies is difficult to establish in such a dense environment.Comment: 16 pages, 3 figures. Accepted for publication in Ap

How to generate pentagonal symmetry using Turing systems

We explore numerically the formation of Turing patterns in a confined circular domain with small aspect ratio. Our results show that stable fivefold patterns are formed over a well defined range of disk sizes, offering a possible mechanism for inducing the fivefold symmetry observed in early development of regular echinoids. Using this pattern as a seed, more complex biological structures can be mimicked, such as the pigmentation pattern of sea urchins and the plate arrangements of the calyxes of primitive camerate crinoids

A three-dimensional view of the remnant of Nova Persei 1901 (GK Per)

We present a kinematical study of the optical ejecta of GK Per. It is based on proper motions measurements of 282 knots from ~20 images spanning 25 years. Doppler-shifts are also computed for 217 knots. The combination of proper motions and radial velocities allows a unique 3-D view of the ejecta to be obtained. The main results are: (1) the outflow is a thick shell in which knots expand with a significant range of velocities, mostly between 600 and 1000 km/s; (2) kinematical ages indicate that knots have suffered only a modest deceleration since their ejection a century ago; (3) no evidence for anisotropy in the expansion rate is found; (4) velocity vectors are generally aligned along the radial direction but a symmetric pattern of non-radial velocities is also observed at specific directions; (5) the total Halpha+[NII] flux has been linearly decreasing at a rate of 2.6 % per year in the last decade. The Eastern nebular side is fading at a slower rate than the Western one. Some of the knots displayed a rapid change of brightness during the 2004-2011 period. Over a longer timescale, a progressive circularization and homogenization of the nebula is taking place; (6) a kinematic distance of 400+-30 pc is determined. These results raise some problems to the previous interpretations of the evolution of GK Per. In particular, the idea of a strong interaction of the outflow with the surrounding medium in the Southwest quadrant is not supported by our data.Comment: Accepted for publication in The Astrophysical Journal (19 pages, 17 figures). Higher resolution version of this article (2.5 MB) is available at http://www.aai.ee/~sinope/ApJ89291_liimets.pd

Tracking Curvaton(s)?

The ratio of the curvaton energy density to that of the dominant component of the background sources may be constant during a significant period in the evolution of the Universe. The possibility of having tracking curvatons, whose decay occurs prior to the nucleosynthesis epoch, is studied. It is argued that the tracking curvaton dynamics is disfavoured since the value of the curvature perturbations prior to curvaton decay is smaller than the value required by observations. It is also argued, in a related context, that the minimal inflationary curvature scale compatible with the curvaton paradigm may be lowered in the case of low-scale quintessential inflation.Comment: 20 pages, 4figure

Weber blockade theory of magnetoresistance oscillations in superconducting strips

Recent experiments on the conductance of thin, narrow superconducting strips have found periodic fluctuations, as a function of the perpendicular magnetic field, with a period corresponding to approximately two flux quanta per strip area [A. Johansson et al., Phys. Rev. Lett. {\bf 95}, 116805 (2005)]. We argue that the low-energy degrees of freedom responsible for dissipation correspond to vortex motion. Using vortex/charge duality, we show that the superconducting strip behaves as the dual of a quantum dot, with the vortices, magnetic field, and bias current respectively playing the roles of the electrons, gate voltage and source-drain voltage. In the bias-current vs. magnetic-field plane, the strip conductance displays what we term `Weber blockade' diamonds, with vortex conductance maxima (i.e., electrical resistance maxima) that, at small bias-currents, correspond to the fields at which strip states of $N$ and $N+1$ vortices have equal energy.Comment: 4+a bit pages, 3 figures, 1 tabl

Production cross-sections and momentum distributions of fragments from neutron-deficient 36Ar at 1.05 A.GeV

We have measured production cross sections and longitudinal momentum distributions of fragments from neutron-deficient 36Ar at 1.05 A.GeV. The production cross-sections show excellent agreement with the predictions of the semiempirical formula EPAX. We have compared these results, involving extremly neutron deficient nuclei, with model calculations to extract informa tion about the response of these models close to the driplines. The longitudinal momentum distributions have also been extracted and are compared with the Goldhaber and Morrissey systematics.Comment: 16 pages, 6 figure